A conventional cold cathode tube lighting apparatus 50 shown in FIG. 1 typically consists of a chopper circuit 51 to control a cold cathode tube current IL to a predetermined value, a parallel resonance circuit 52 composed of a transformer and a capacitor, a ballast capacitor C5 to stabilize discharge, a tube current sensing circuit 14, and a control circuit 53 to control a power supply period for the chopper circuit 51.
To reduce the size of the transformer as small as possible, a turn ratio n thereof is set to n={V(STRIKE)}/(2πVIN(DC), where V(STRIKE) (see FIG. 2) is a lighting start voltage of a cold cathode tube 8. It is usual to set a maximum output voltage of the secondary side of the transformer to V(STRIKE).